Diodes are used in a wide range of electronic circuits. Diodes used in circuits for high voltage switching applications ideally require the following characteristics. When biased in the reverse direction (i.e., the cathode is at a higher voltage than the anode), the diode should be able to support a large voltage while allowing as little current as possible to pass through. The amount of voltage that must be supported depends on the application; for example, many high power switching applications require diodes that can support a reverse bias of at least 600V or at least 1200V without passing a substantial amount of current. When current flows through the diode in the forward direction (from anode to cathode), the forward voltage drop across the diode Von should be as small as possible to minimize conduction losses, or in other words the diode on-resistance Ron should be as small as possible. Finally, the amount of charge stored in the diode when it is reverse biased should be as small as possible to reduce transient currents in the circuit when the voltage across the diode changes, thereby reducing switching losses.
In diodes, there is typically a trade-off between the various characteristics described above. For example, Silicon Schottky diodes can typically exhibit excellent switching speed and on-state performance but suffer from large reverse leakage currents, making them unsuitable for high voltage applications. Conversely, high voltage Si PIN diodes can support large reverse bias voltages with low leakage but typically exhibit high conduction and switching losses. Further, reverse recovery currents in PIN diodes add to transistor losses in circuits.
Illustrations of typical Schottky diodes are show in FIGS. 1 and 2. FIG. 1 shows a vertical diode structure. Layers 2 and 4 are comprised of a semiconductor material of the same conductivity type, wherein layer 2 is heavily doped and layer 4 is lightly doped. Metal layer 7 forms a Schottky anode contact to layer 4, and metal layer 8 forms an ohmic cathode contact to layer 2. Increasing the active device area and/or decreasing the thickness of semiconductor layer 4 reduces the forward operating voltage Von but increases reverse-bias leakage.
FIGS. 2a and 2b show a lateral diode structure, wherein FIG. 2a is a cross-sectional view and FIG. 2b is a plan view (top view) of the diode structure. Layers 12 and 14 are comprised of a semiconductor material of the same conductivity type (i.e., they are either both n-type or both p-type), wherein layer 12 is heavily doped and layer 14 is lightly doped. Metal layer 17 forms a Schottky contact to layer 14, and metal layer 18 forms an ohmic contact to layer 2. This geometry can be preferable to a vertical one when a planar structure for the anode and cathode is required for packaging, or when the semiconductor material is epitaxially grown on an insulating substrate. The on-resistance Ron for the lateral geometry is typically larger than that for the vertical geometry due to the added lateral resistance of region 19 through which the forward current must travel. Additionally, as a result of the forward current flowing laterally outwards through a layer 12 with non-zero sheet resistance, the current through 14 tends to crowd towards the edges of the mesa, thereby further increasing the on-resistance.
In standard Schottky diodes, Schottky barrier lowering occurs when the diode is reverse biased, resulting in increased reverse bias currents. Schottky barrier lowering for the diode in FIG. 1 is illustrated schematically in the diagrams of FIGS. 3a and 3b. FIGS. 3a and 3b are band diagrams along dotted line 117 in FIG. 1, where FIG. 3a corresponds to zero applied bias, i.e. anode contact 7 and cathode contact 8 are at the same voltage, and FIG. 3b is for a reverse bias VR, i.e. anode contact 7 is at a lower voltage than the cathode contact 8. The electric field in the structure is proportional to the slope of the conduction band EC in FIGS. 3a and 3b. The Schottky barrier height in FIG. 3b, (ΦB)R, is less than that in FIG. 3a, (ΦB)0, by an amount ΔΦB, where ΔΦB increases as the maximum electric field near the metal-semiconductor junction increases, which occurs when VR is increased. This lowering of the Schottky barrier results in increased reverse bias currents as the reverse voltage across the device is increased.
It is desirable to provide diodes for which high blocking voltages can be achieved while at the same time maintaining lower on-resistances. Diode structures which can easily be integrated with other circuit components, such as transistors, are desirable for process integration and cost reduction. Additionally, Schottky diodes for which Schottky barrier lowering is mitigated are desirable, since lower reverse leakage currents can potentially be achieved.